Apple on Friday issued an open letter to iPhone 4 users, revealing that the formula used to calculate bars of signal strength on the device is inaccurate, and will be corrected in the coming weeks through a free software fix.

The letter confirms what two in-depth takes released this week had already stated: that there is a flaw in how Apple's iOS mobile operating system reports bars of signal strength.

Apple's iOS allots nearly 40 percent of reception levels to five bars, from -51 dB to -91 dB. But the distance from four bars to one bar of reception is much less, from -91 dB to -113 dB.

"Our formula, in many instances, mistakenly displays 2 more bars than it should for a given signal strength," Apple's letter reads. "For example, we sometimes display 4 bars when we should be displaying as few as 2 bars. Users observing a drop of several bars when they grip their iPhone in a certain way are most likely in an area with very weak signal strength, but they don't know it because we are erroneously displaying 4 or 5 bars."

A software fix due to be released in the coming weeks will adopt AT&T's guidelines for signal strength reporting, which will result in a more accurate portrayal of reception on the iPhone 4.

"We have gone back to our labs and retested everything, and the results are the same the iPhone 4's wireless performance is the best we have ever shipped," the letter states. "For the vast majority of users who have not been troubled by this issue, this software update will only make your bars more accurate. For those who have had concerns, we apologize for any anxiety we may have caused."

Testing of the iPhone 4 found that holding it in the left hand and covering the left side of the device can reduce reception by 24 dB. For a user with a "weak," incorrectly reported five bars, this can result in a dramatic reduction of bars on the iPhone 4.

Apple's software update plans to correct this issue, and more accurately report the signal degradation from holding the device -- something the company said occurs with any phone, including the iPhone 3GS, and phones from competitors like Nokia, Research in Motion, and those running Google's Android mobile operating system.

Included is Apple's letter in its entirety:

Dear iPhone 4 Users,

The iPhone 4 has been the most successful product launch in Apple's history. It has been judged by reviewers around the world to be the best smartphone ever, and users have told us that they love it. So we were surprised when we read reports of reception problems, and we immediately began investigating them. Here is what we have learned.

To start with, gripping almost any mobile phone in certain ways will reduce its reception by 1 or more bars. This is true of iPhone 4, iPhone 3GS, as well as many Droid, Nokia and RIM phones. But some users have reported that iPhone 4 can drop 4 or 5 bars when tightly held in a way which covers the black strip in the lower left corner of the metal band. This is a far bigger drop than normal, and as a result some have accused the iPhone 4 of having a faulty antenna design.

At the same time, we continue to read articles and receive hundreds of emails from users saying that iPhone 4 reception is better than the iPhone 3GS. They are delighted. This matches our own experience and testing. What can explain all of this?

We have discovered the cause of this dramatic drop in bars, and it is both simple and surprising.

Upon investigation, we were stunned to find that the formula we use to calculate how many bars of signal strength to display is totally wrong. Our formula, in many instances, mistakenly displays 2 more bars than it should for a given signal strength. For example, we sometimes display 4 bars when we should be displaying as few as 2 bars. Users observing a drop of several bars when they grip their iPhone in a certain way are most likely in an area with very weak signal strength, but they don't know it because we are erroneously displaying 4 or 5 bars. Their big drop in bars is because their high bars were never real in the first place.

To fix this, we are adopting AT&T's recently recommended formula for calculating how many bars to display for a given signal strength. The real signal strength remains the same, but the iPhone's bars will report it far more accurately, providing users a much better indication of the reception they will get in a given area. We are also making bars 1, 2 and 3 a bit taller so they will be easier to see.

We will issue a free software update within a few weeks that incorporates the corrected formula. Since this mistake has been present since the original iPhone, this software update will also be available for the iPhone 3GS and iPhone 3G.

We have gone back to our labs and retested everything, and the results are the same the iPhone 4's wireless performance is the best we have ever shipped. For the vast majority of users who have not been troubled by this issue, this software update will only make your bars more accurate. For those who have had concerns, we apologize for any anxiety we may have caused.

As a reminder, if you are not fully satisfied, you can return your undamaged iPhone to any Apple Retail Store or the online Apple Store within 30 days of purchase for a full refund.

This is exactly what I suspected all along but I expect those screaming it is a major issue to claim this is an excuse. All reports show the iPhone 4 has better signal capture than the 3Gs so the issues with the bars is obviously letting users venture into weaker signal areas, add in a falsely high bar reading then add in a hand over the antennae and voila ... the hysteria!

From Apple ][ - to new Mac Pro I've owned them all.Long on AAPL so biased"Google doesn't sell you anything, Google just sells you!"

If, with the corrected signal bar reading, you move to an area of good signal then assuming the other end or tower has no issues then it should stop most drops. The issue now is users are being lulled into thinking they have a good signal when they don't. At that point blocking the signal is pretty easy whereas in a stronger signal area the blocking won't occur to the extent it drops a call.

From Apple ][ - to new Mac Pro I've owned them all.Long on AAPL so biased"Google doesn't sell you anything, Google just sells you!"

Yes, if indeed the exaggerated bars were a deliberate ploy then it backfired badly but I doubt it was intentional as the consequences of fake high readings are pretty obvious to anyone who thinks it through.

From Apple ][ - to new Mac Pro I've owned them all.Long on AAPL so biased"Google doesn't sell you anything, Google just sells you!"

This is exactly what I suspected all along but I expect those screaming it is a major issue to claim this is an excuse. All reports show the iPhone 4 has better signal capture than the 3Gs so the issues with the bars is obviously letting users venture into weaker signal areas, add in a falsely high bar reading then add in a hand over the antennae and voila ... the hysteria!

Judging by my experience this makes a lot of sense. I get poor reception in my office and my 3G and 3Gs (for the 2 weeks I had it) would hover around 1 or 2 bars all day. I've noticed my 4 fluctuating from 5 to 2 to 4 to 1 while just laying flat on my desk. And even though I'm in a weak area I haven't been able to get it to drop to no service or searching depending on how it's held

Judging by my experience this makes a lot of sense. I get poor reception in my office and my 3G and 3Gs (for the 2 weeks I had it) would hover around 1 or 2 bars all day. I've noticed my 4 fluctuating from 5 to 2 to 4 to 1 while just laying flat on my desk. And even though I'm in a weak area I haven't been able to get it to drop to no service or searching depending on how it's held

Yep same here. It is a shame this ever happened though, who ever did the bar software should be kicked hard up the butt. It has given the Apple haters way too much fodder ... although sales were not too shabby anyway

From Apple ][ - to new Mac Pro I've owned them all.Long on AAPL so biased"Google doesn't sell you anything, Google just sells you!"

It's good to have confirmation from Apple, but it isn't like this isn't exactly what Anand reported. The algorithm for the bars display is weighed to show a better signal than actually existed. The first problem of holding it resulting in the dropping of the number of bars being displayed is mainly a visualization problem..while there is a signal drop, it is not as pronounced as the display indicates.

But as Anand also stated, it doesn't change the fact that the actual amount of signal loss is still a problem.:
"The drop in signal from holding the phone with your left hand arguably remains a problem. Changing the bars visualization may indeed help mask it, and to be fair the phone works fine all the way down to -113 dBm, but it will persist - software updates can change physics as much as they can change hardware design. At the end of the day, Apple should add an insulative coating to the stainless steel band, or subsidize bumper cases. It's that simple."

"My 8th grade math teacher once said: "You can't help it if you're dumb, you are born that way. But stupid is self inflicted."" -Hiro.

I finally gave up the ghost and coughed up $150 to AT&T for a MicroCell. For what it's worth, my iPhone now works great at home! Has my fanboyism gotten the better of me? Perhaps, but I'm happy and that's what matters.

Yep same here. It is a shame this ever happened though, who ever did the bar software should be kicked hard up the butt. It has given the Apple haters way too much fodder ... although sales were not too shabby anyway

Even smartest people make some very stupid mistakes, actually you can call it a rule.

If, with the corrected signal bar reading, you move to an area of good signal then assuming the other end or tower has no issues then it should stop most drops. The issue now is users are being lulled into thinking they have a good signal when they don't. At that point blocking the signal is pretty easy whereas in a stronger signal area the blocking won't occur to the extent it drops a call.

As the report by AnandTech said that iPhone 4 has better signal reception, I think the update will lower the limit of "no signal".

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It's good to have confirmation from Apple, but it isn't like this isn't exactly what Anand reported. The algorithm for the bars display is weighed to show a better signal than actually existed. The first problem of holding it resulting in the dropping of the number of bars being displayed is mainly a visualization problem..while there is a signal drop, it is not as pronounced as the display indicates.

But as Anand also stated, it doesn't change the fact that the actual amount of signal loss is still a problem.:
"The drop in signal from holding the phone with your left hand arguably remains a problem. Changing the bars visualization may indeed help mask it, and to be fair the phone works fine all the way down to -113 dBm, but it will persist - software updates can change physics as much as they can change hardware design. At the end of the day, Apple should add an insulative coating to the stainless steel band, or subsidize bumper cases. It's that simple."

The amount of signal drop is a fixed constant as it relates to the persons hand and position (let us assume this for the sake of simplicity). So there comes point where the signal is over powered by this blocking totally. Once you move to where there is enough signal to over come the block you are OK. In the area right on the point where the signal and blocking are equal the wavering signal strength will fluctuate just above and just below the blocking.

So I agree the bumper or clear coat would help for these marginal areas which, if this is your office, becomes very important. However, the same could be said for every mobile phone in existence. Maybe the iPhone 4 now has yet another advantage, we all know now how to add that extra drop of reception

From Apple ][ - to new Mac Pro I've owned them all.Long on AAPL so biased"Google doesn't sell you anything, Google just sells you!"

So basically they are saying that they're going to update your phone to tell you that your reception is worse than it really is, bar wise. This doesn't fix anything. This isn't going to fix the people who live in lower reception areas from losing service completely or drop low enough to drop calls, where their previous iPhones or whatever they used works fine.

This is just smoke and mirrors. They also need to address the proximity sensor, although I think they are going to sneak it in. It's a real simple fix, just give it more tolerance.

I've noticed that the bars sometimes seem to be showing more bars than I would expect however the simple fact still remains that I can download 3G radio streams with no issues and then lightly hold the phone in my left hand and drop out of 3G to to the slower network and then get no service all within 10 seconds or so. And Apple thinks that normal!

"Islam is as dangerous in a man as rabies in a dog"~ Sir Winston Churchill. We are nurturing a nightmare that will haunt our children, and kill theirs.

So basically, Apple is saying there is no problem with the antena, the only issue is you thought you were getting better reception thatn you were?

Even more BS that they say they've always calculated things this way. This means people should not be seeing any worse problems with the 4 than the earlier 3. Evidence does not bear this out.

This makes no sense. People's calls are dropping and data speed is much slower when portions of the antenna and touched and/or bridged. Apple's solution is to show you that you have less signal all the time?

I know all phones lose some signal when the antenna is covered, but APPLE'S IS THE ONLY EXTERNAL ANTENNA TOTALLY EXPOSED TO SKIN ETC.

If this indeed Apple's solution, ie. screw you, then I suspect they'll get a lot of phones back. When mine comes, if I have this issue I will take the opportunity to return it and go Android (not what I want to do). Apple is still in denial there there is a problem.

So basically they are saying that they're going to update your phone to tell you that your reception is worse than it really it, bar wise. This doesn't fix anything. This isn't going to fix the people who live in lower reception areas from losing service completely or drop low enough to drop calls, where their previous iPhones or whatever they used works fine.

This is just smoke and mirrors. They also need to address the proximity sensor, although I think they are going to sneak it in. It's a real simple fix, just give it more tolerance.

Sorry, but you have your logic totally backward. It will not tell you the signal is worse that it really is, it will be accurate. The initial software reported a signal where there was no signal hence the frustration. Once it is accurate you will know where to get a call through ... like it has always been with mobile phones since they first came out ...

From Apple ][ - to new Mac Pro I've owned them all.Long on AAPL so biased"Google doesn't sell you anything, Google just sells you!"

It's good to have confirmation from Apple, but it isn't like this isn't exactly what Anand reported. The algorithm for the bars display is weighed to show a better signal than actually existed. The first problem of holding it resulting in the dropping of the number of bars being displayed is mainly a visualization problem..while there is a signal drop, it is not as pronounced as the display indicates.

But as Anand also stated, it doesn't change the fact that the actual amount of signal loss is still a problem.:
"The drop in signal from holding the phone with your left hand arguably remains a problem. Changing the bars visualization may indeed help mask it, and to be fair the phone works fine all the way down to -113 dBm, but it will persist - software updates can change physics as much as they can change hardware design. At the end of the day, Apple should add an insulative coating to the stainless steel band, or subsidize bumper cases. It's that simple."

Indeed. It also seems that people are missing the part of the letter where it states that this "mistake" in reporting the bars as stronger than they are, has been going on since the first iPhone came out. In other words the iPhone has always reported it's bars this way, and only with the new physical hardware design has it turned out to be a problem. This is tantamount to confirmation from Apple that the iPhone 4 actually has a hardware/design problem.

It is ok to love a phone and still be dissatisfied with a portion of it's performance. It is fair to complain.

I'm fine with the signal my self but I wish the notifications were more like on my Pre. There. I am keeping the phone and love it, but I found something to complain about.

Not all of us worship at the alter.

That makes sense, when the portion of the performance you're not satisfied with is not the PHONE part. That's like saying buying a car that you don't like the radio layout is the same thing as buyign a car that doesn't drive or handle properly.

So how does this explain the testers who could make calls from their desk on the 3G S with no problem, but who get dropped calls from the same place using the 4G?

These reports don't stack up to well from anyone I have spoken to. Color me skeptical on many of these reports. Of course there are always exceptions but don't discount hysteria.

Humans are funny creatures. If I tell a client I have altered some code in their web site often they suddenly get problems, they are convinced are due to the changes ... even when I haven't made any changes (I fibbed)... I do this sort of thing to test people's sanity from time to time and alleviate boredom

From Apple ][ - to new Mac Pro I've owned them all.Long on AAPL so biased"Google doesn't sell you anything, Google just sells you!"